Geared steering devices for a crawler vehicle have been proposed, for example, in the Published Japanese Translations of PCT Patent Applications from Other Countries No. 501139-1980. As shown in FIG. 5, a vehicle 1 having geared steering devices mounted thereon has a travel device comprising a rightside track shoe 2 and a leftside track shoe 3. Each of these track shoes 2, 3 is driven by a respective one of drive sprockets 6R, 6L which are connected to a prime mover driving device 7, such as an internal combustion engine. Specifically, as shown in FIG. 6, a driving power of the driving device 7 is transmitted to each drive sprocket 6R and 6L through an input shaft 9, and respective ones of geared steering devices 8R, 8L, output shafts 10R, 10L, and final reduction gears 4R, 4L.
The operations of conventional geared steering devices 8R, 8L in a high speed mode, a low speed mode, and a braking mode will be described with reference to a skeletonized illustration thereof in FIG. 12.
In the high speed mode, brakes 11R, 11L and low speed clutches 12R, 12L are released, and the rotatable housings 14R, 14L are coupled to drive hubs 15R, 15L by engaging high speed clutches 13R, 13L. Accordingly, the input shaft 9 is directly gear-engaged with housings 14R, 14L, and the output shafts 10R, 10L are directly gear-engaged with the brake hubs 22R, 22L. Therefore, the driving force of a prime mover 7 directly drives the output shafts 10R, 10L by means of the input shaft 9 through bevel gears 23, 24 so that the rotation of the input shaft 9 is transmitted to the output shafts 10R, 10L at the speed ratio of 1 to 1.
In the low speed mode, although the high speed clutches 13R, 13L and brakes 11R, 11L are released, the hubs 16R, 16L are secured to the fixed housing 17 by the engagement of the low speed clutches 12R, 12L, and the sun gears 18R, 18L are also fixed. Therefore, the planet gears 19R, 19L move around the sun gears 18R, 18L when the planet gears 19R, 19L are driven by the ring gears 20R, 20L, which are attached to input housings 14R, 14L. As it is common to utilize a plurality of planet gears rotatably mounted on a common carrier, the planet gears of a planet gear train will be referred to collectively as a carrier gear. With this arrangement, carriers gears 21R, 21L rotate at a speed reduced by a specified reduction gear ratio, and the output shafts 10R, 10L also rotate at the same rotation rate through the drive hub 15R, 15L being directly coupled to carrier gears 21R, 21L.
In the braking mode, the high speed clutches 13R, 13L and the low speed clutches 12R, 12L are released. Therefore, the torque transmission from the input shaft 9 to the output shafts 10R, 10L is interrupted. Simultaneously, brakes 11R, 11L are engaged, and therefore brake hubs 22R, 22L directly couple the output shafts 10R, 10L to the fixed housing 17. Thus, the output shafts 10R, 10L and the related track shoes 2, 3 are braked.
A control unit of a vehicle having a conventional clutch/brake type steering device has been proposed in the Published Japanese Translations of PCT Patent Applications from Other Countries No. 501054-1989. Specifically, this reference has disclosed a technology for a feedback control of a hydraulic pressure of the clutches or brakes so that a designated turning radius of the vehicle, which is determined in accordance with an amount of operation of a steering lever, may be obtained by sensing the rotation rates of rightside and leftside track shoe drive shafts. Such control unit is free from braking loss, owing to the specified gear ratio, and enables the vehicle to turn in a turning radius of approximately 10 m. It is, of course, provided with the functions of the known clutch/brake type turning device which depend on brake operation.
However, for such control unit a total of six discs has been required for the high speed clutches 13R, 13L, the low speed clutches 12R, 12L, and the brakes 11R, 11L. Similarly, a total of six sets of hydraulic control valves has been required for the rightside and leftside control mechanisms. Therefore, the control unit would be expensive. The vehicle is provided with the high speed clutches 13R, 13L and the low speed clutches 12R, 12L at its rightside and its leftside, and therefore a high speed stage or a low speed stage can be provided both at the rightside and at the leftside of the vehicle. Accordingly, if a three stage transmission is provided, the forward and rearward vehicle speeds can be shifted in six steps. However, such transmission system would be excessive for a vehicle for which a three stage transmission system is sufficient.
On the other hand, a control unit of a vehicle having geared steering devices 8R, 8L requires the capability of obtaining a designated turning radius of the vehicle which is determined with an operating amount of the steering lever. Unless the feedback control is given, there may be a risk of steering opposite to an operation of the operator in accordance with a degree of load which acts on the vehicle. However, such control technique has not yet been disclosed.